This work was supported, in part, by funds from the UNDP/World Bank/WHO special Program for Research and Training in Tropical Disease (grant ID 990 806 and IDA10124), and Grants-in Aid for Scientific Research (A) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. A part of work was supported by the 21st Century COE Program, Ministry of Education, Culture, Sports, Science and Technology (MEXT)

Abstract

In our investigation of in vitro antimalarial screening of medicinal herbal extracts, the n-BuOH extract from the root of Wikstroemia indica showed a potent inhibitory effect. Fractionation of the active extract led to the isolation of two biflavonoids, sikokianin B (1) and sikokianin C (2) with IC50 values 0.54 μg/mL and 0.56 μg/mL, respectively, against the chloroquine-resistant strain of Plasmodium falciparum. This is the first report of the biological activity of 1 and 2. As the structure of l has remained unsettled, we confirmed the conformation by 1H- and 13C-NMR.

To discover antimalarial substances from medicinal herbs, EtOAc-, n-BuOH- and H2O-soluble fractions were prepared from the initial EtOH/H2O (1 : 1) extracts. These were then screened in vitro against the chloroquine-resistant K1 strain of Plasmodium falciparum. This revealed that the n-BuOH-soluble fraction of the root of Wikstroemia indica (Linne) C. A. Meyer (Thymelaeaceae) had appreciable antimalarial inhibitory activity (Table [1]).

The n-BuOH extract was then subjected to activity-guided purification by column chromatography on silica gel, followed by medium pressure liquid chromatography (MPLC), which gave three fractions (fr.1, fr.2 and fr.3). Compounds 1 and 2 were obtained from fr.1 and fr.2, respectively, in a pure form by repeated MPLC. Each purification step enhanced the antimalarial potency, with the IC50 values of 1 and 2 being determined as 0.54 μg/mL and 0.56 μg/mL (Table [1]), respectively. Compounds 1 and 2 showed almost similar activity with chloroquine, but they had one fifty-seventh activity compared with artemisinin.

Compound 1 was obtained as an amorphous powder with optical activity ([α]D30: + 199.7°), showed a molecular ion in the HR-FAB-MS at m/z = 557.1448 [M + H]+, indicating a molecular formula of C31H24O10. In the 1H- and 13C-NMR edited by 1H-1H COSY and HMQC experiments (Table [2]), the spectra showed its structural fragments to include two 1,2,3,5-tetrasubstituted benzenes (C-5 to C-10 and C-5′′ to C-10′′), two 1,4-disubstituted benzenes (C-1′ to C-6′ and C-1′′′ to C-6′′′), one 1,2,3,4-tetrasubstituted n-butyl group (C-2, C-3, C-2′′ and C-3′′), one methoxy group and two carbonyls (C-4 and C-4′′). These structural fragments were connected to form the given carbon framework of 1 by HMBC, NOESY and LSPD (Long Range Selective Proton Decoupling) spectra, and the structure was shown to be a dimer of flavanonol derivatives which were connected C-3 (δC = 49.5) to C-3′′ (δC = 50.8). From the 1H-1H COSY spectra, the stereochemistry at C-2 (δH = 5.17, d, J = 9.0Hz)/C-3 (δH = 3.33, dd, J = 9.0, 3.5Hz), C-3/C-3′′ (δH = 3.23, dd, J = 3.5, 3.5Hz) and C-2′′ (δH = 5.52, d, J = 3.5Hz)/ C-3′′ positions exhibited trans, cis and cis geometry, respectively. Further from the NOESY spectra, significant spatial conjugations between H-2/H-2′′′ (H-6′′′) were observed. The relative stereochemistry of compound 1 was confirmed as shown in Fig. [1].

Since the compound 1 was previously reported as sikokianin B [1] of which the chirality at the C-3/C-3′′ position and the detailed assignment by NMR were unsettled, we assigned and confirmed the structure by the spectral data. Analysis of the spectral data showed compound 2 to be sikokianin C [2] (Fig. [1]). No previous reports have appeared for the isolation of 1 and 2 from the root of W. indica or of them having antimalarial activity. The root of W. indica has been used for the treatment of scrofula, rheumatalgia, carbuncle, traumatic injury, etc. in China [3] but it is not used for malaria.

Compounds 1 and 2 were assayed against the drug-sensitive FCR3 P. falciparum strain, with the resulting IC50 values of 0.54 μg/mL and 0.34 μg/mL, respectively. The activity of 1 and 2 against both the K1 and FCR3 strains was similar suggesting no cross-resistance with chloroquine. Compounds 1 and 2 showed the selectivity indexes (cytotoxicity [IC50 for the MRC-5 cells]/antimalarial activity [IC50 for the K1 strain]) with the ratios of 41.7 and 20.0, respectively. The results discussed above contribute to a growing list of bioactive compounds obtained from natural sources and as such may provide lead compounds for synthesis of more effective antimalarials.

Fig. 1 Biflavonoids from the root of W. indica

Table 1 Inhibitory effects of fractions and compounds from W. indica for antimalarial activity against K1, FCR3 and cytotoxicity against MRC-5 cells

Materials and Methods

Optical rotations were measured with a JASCO polarimeter at 30 °C. 1H- and 13C-NMR spectra were determined on a Varian Unity 400 machine. Mass spectra (MS) were obtained on a JEOL MXA-AM505HA spectrometer.

Plant material: The root of Wikstroemia indica was purchased under the herbal name of ”Ryokao” in Japanese (Liao-ge-wang in Chinese) from Yamamoto Yakuhin Kogyo Co., Ltd (Tokyo) in April, 2001, and the botanical origin of Ryokao was identified by Dr. Shinyu Nunome in Kitasato Institute for Life Sciences. The voucher specimen (KT-280) has been deposited at the Herbarium of the Kitasato Institute for Life Sciences of Kitasato University.

Antimalarial activity and cytotoxicity: The assays were performed as described in the previous paper [4]. Antimalarial assays were conducted using the drug-resistant K1 strain and the drug-sensitive FCR3 strain of Plasmodium falciparum. Chloroquine and artemisinin were used as positive controls. Cytotoxicity was assayed against human diploid embryonic cell line MRC-5.